Prep: Preheat your oven to 375°F (190°C). Line a 12-cup muffin tin with paper liners.
Dry Mix: In a medium bowl, whisk together 1 ½ cups all-purpose flour, 1 tsp baking soda, ½ tsp baking powder, and ½ tsp fine sea salt.
Wet Mix: In a large bowl, whisk the 3 mashed bananas, 6 tbsp melted butter, ½ cup brown sugar, ¼ cup white sugar, 1 egg, and 1 tsp vanilla extract until smooth.
Combine: Pour the flour mixture into the banana mixture. Using a spatula, fold gently until just a few streaks of dry flour remain.
Fold: Gently stir in ¾ cup of the chocolate chips. Be careful not to overmix; stop as soon as the chips are distributed.
Fill: Divide the batter evenly among the 12 muffin cups. Sprinkle the remaining ¼ cup of chocolate chips over the tops.
Bake: Bake for 18–20 minutes at 375°F, or until a toothpick inserted into the center comes out clean.
Cool: Let the muffins sit in the tin for 5 minutes, then move them to a wire rack to finish cooling.